The demand for decontaminated fresh water has been steadily increasing in the United States and world wide due to increasing populations and increasing industrialization, mining operations, and agriculture and this trend is projected to continue. In addition, fresh water sources, such as wells that use aquifer water are increasingly found to be contaminated by these mostly human activities. A particularly problematic example is that of mining, especially that of coal mining, where the mining activity has exposed gangue minerals left in the mine and mine tailings to erosion by air, water, and microbial action. Sulfidic minerals, such as pyrite, FeS2, are commonly found in many geological strata, and especially in reducing ore bodies such as coals and metal sulfide.
As described thoroughly in an extensive literature spanning many decades, coal, metal and other mining operations, and natural weathering fissures have allowed water, air, and microbial access to these reducing substances. These conditions promote the oxidation of the sulfidic minerals to water soluble ferrous sulfate and other metal sulfates; thereby, especially when exposed to air at the surface, producing an acidic discharge or ground water of hundreds and even often above 2000 ppm of total dissolved solids (see Tables 2A, 2B, and 2C). Such contaminated ground water is unsuitable for most uses, including municipal, industrial, residential, and farming; is foul tasting; is odorous; is toxic to aquatic ecosystems, plants, and animals; and can mobilize additional metal contaminants by acid dissolution of natural or man-made materials.
The problems of water purification and treatments of acid mine drainage and acid rock drainage, collectively referred to as acid mine drainage or AMD water, are well described in the literature. Hereinafter, when acid mine drainage is discussed, unless mentioned otherwise, the text will apply to acid rock and natural gas well brines drainage also. Conventional technologies have been found to be unsuitable for processing such waters especially where total flow rates exceed 10 gal/min and can reach 100,000 gal/min. Dissolved metal cation and counter anion concentrations in acid mine drainage can be far above the levels removable by technologies known in the art. Other conventional technologies have major disadvantages including high initial capital costs, slow reaction times, high reagent costs, reagent hazards, and/or production of waste sludges. But most importantly, they do not remove the major problematic contaminant, sulfate ion. Sulfate ion is responsible for heavy fouling or scaling during industrial use, foul tastes and odors, laxative effects, and very high level of corrosiveness to construction metals and concrete.